My own fanon says that Harley is pretty damn sharp, but she doesn't wanna learn what she doesn't care about, so she fudged and flirted and other stuff on the math and languages but soaked up the psych.
You are very far from the only fan to think this, I assure you. :) It's nice to hear it outside the immediate fan circles too, as some 'fans' seem intent on characterising her as stupid. I also think Harley has a natural knack for what makes people tick.